Process to deploy artifacts for multiple platforms into a single release.

Remove useless file that doesn't load

This file was broken in ada65567.

Change-Id: Ib75664194491643f8e4f1503a2ed942a2d1e1655

release.

- Do not close the staging repository automatically
- Added staging.repository property
- Updated README with instructions for deployment
- Fix building 32-bit Mac artifact

perf: String#getBytes(Charset) vs getBytes(String)

Better support for cross-compilation

Build scripts to publish precompiled protoc binaries (Maven-based)

Run make protoc(.exe) instead of just make, because the latter fails on Cygwin with mingw gcc; build-protoc.sh works under Cygwin; build-protoc.sh will use the --host parameter consistent with what os-maven-plugin has detected

Fix raise and except syntax issues for py3

Clean up generated files

This fixes the problem where `make check` generates files not ignored
by `.gitignore`.